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What gives the L36 Higher Compression?

BlackGT97

BlackGT97

New member
I realize there are several different ways to achive higher compression. And seems how I never Got into the internals of the 3.8 (yet), How did GM go about this? And if the connecting rods are different lengths between the L36 and L67, How is the stroke the same? I thought It would have been different stoke lengths.
 


i though it was in the PCM because you can just top swap to an L67 i mean this is a really good question and id love to know the real answer
 
I think its due to the rod length. If it is, then:

Longer rods allow the piston to travel further toward the head, increasing compression. So why doesn't the stroke change? This is because the longer rods don't allow the pistons to receed as far into the bottom of the cylinder as the L67.
 


ive never done a top swap, i though you swaped over the l67 heads up and left the NA block. im just trying to get a better understanding not sound like i know what im talking about
 
asite57 said:
so why can you just swap the top over and call it good?
Click to expand...

You can't topswap and call it good. You have to tune for it. Having the higher compression and Boost causes more heat, which causes detonation. Flow mods, cooler plugs, and cooler air (intercooler) and tuning, are all ways to fight the detonation, AKA knock.
 


As far as I know its the piston.

As in the dish size. Dish CC unknown, but around 10 cc for 8.5 CR and 4-5 cc for 9.4 CR IIRC.

The rod length is related to the different wrist pin location in the piston. Somewhere in the mid low 5" "total length", center-to-center is under 5" IIRC.

The CC in the heads are the same too. At one point the casting was the same with some extra machine work to add the injector bungs for the L67. 64 cc IIRC.

Only absolute number here I'm sure of is the combustion chamber number from intense.

On the top swap concept: you'll need headers to run a 3.8 pulley.
 
asite57 said:
ive never done a top swap, i though you swaped over the l67 heads up and left the NA block. im just trying to get a better understanding not sound like i know what im talking about
Click to expand...
The heads, in the mechanicale aspect are the same. Only thing different is the port for the injectors. So your Pushrods, lifters, open the valves just as far as they would with the N/A heads.
 
mechguy said:
I think its due to the rod length. If it is, then:

Longer rods allow the piston to travel further toward the head, increasing compression. So why doesn't the stroke change? This is because the longer rods don't allow the pistons to receed as far into the bottom of the cylinder as the L67.
Click to expand...

As long as the stroke doesn't change then that doesn't change the compression. You will always start at 0 PSI on the intake stroke. Then no matter where the pistons stops if it makes the same stroke then it should make the same compression. Or my logic could be wrong.
 
Iceman6669 said:
As long as the stroke doesn't change then that doesn't change the compression. You will always start at 0 PSI on the intake stroke. Then no matter where the pistons stops if it makes the same stroke then it should make the same compression. Or my logic could be wrong.
Click to expand...


No sir.

Imagine this: A 10 foot long cylinder with the same stroke as your engine. There would be almost no compression in the former.
 


Piston wrist pin to top of piston length is different. L67/L32's have more valve clearance.

The rods may be same length but the wrist pin ends are different and specific to the piston.
 
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